Textile machines of this type, in particular circular and flat bed knitting machines, are known for example from the DE 15 85 206 A2, DE 20 10 973 A2, DE 21 50 360 A1, DE 25 19 896 A1 and DE 36 14 220 A2. Arrangements of this type having controllable working elements and an electromagnetic control system are especially envisaged for the purpose of controlling patterns and as needle selection devices. Therein, the selection of a needle results from the direct effect upon the individual knitting elements, such as needles, sinkers, springs or the like, or, it is effected by controlling the ferromagnetic parts of the knitting elements indicated, whereby the electromagnetic control system moves relative to the knitting elements located in the needle channels, in a direction which is transverse to that in which the needles are driven-out. In order for the working elements to be got into the requisite initial position for the selection process, there are provided permanent magnets which--by virtue of the effect of cams, if need be--hold the working elements in their initial position for the selection process against the spring force of the take-off springs. Consequently, the permanent magnet, which is disposed in the movable control system, slides over the working elements. In order to reduce the high degree of friction arising thereby, a rail consisting of a material which is as smooth and hard as possible is placed between the poles and the working elements in order to minimise the frictional forces and the abrasion due to the friction. The permanent magnet thereby slides along, over the working elements, until the control system, including its selection system, arrives at the knitting element which it is intended to select. This selection region is as wide as the working element and is separated by a narrow gap from the poles of the permanent magnet so that the magnetic field of the permanent magnet can be neutralised in this region by means of a control coil. Consequently, the selected working element is drawn out by the selection system under the influence of the spring force of a take-off spring, while the other working elements continue to be held in position by the magnetic poles of the permanent magnet.
The magnetic flux in the working elements is dependent on the number of working elements which are respectively selected, or, which are still adhering to the selection system so that the electromagnetic force of attraction that is effective on the working elements also alters. The magnetic flux of the control coil, with which the permanent magnet is compensated in the selection region, also has to be altered in a corresponding manner. It is known from the DE 36 14 220 A2 to provide a Hall probe for measuring the instantaneous magnetic flux and to alter the current being admitted into the control coil of the electromagnet in dependence on the output signal of the Hall probe.
As already mentioned, in all of these conventional selection systems, there arises a considerable amount of friction between the working elements, perhaps the sinkers, and the control system sliding thereover and this friction has an adverse effect upon the reliability of the selection system and the whole manner of operation of the textile machine which becomes worse, the higher the speed of operation. Since the friction is dependent upon the force of attraction i.e. upon the density of the magnet field in the working element, as well as upon the dimensions of the contacting surfaces, one had to attach great importance to keeping the friction small by using as small a magnetic flux as possible in the conventional textile machines of this type. However, the consequence of this was that the working elements were not always held reliably in the selection system, or, that working elements, which were not deliberately selected, became detached from the selection system.
A further substantial disadvantage of conventional selection systems of this type consists in that there is only a short period of time available during which the selection region is magnetically neutral due to the small region in which the selection is carried out. In accordance with a proposal known from the DE-25 56 840 A1, a plurality of selection arrangements are arranged adjacent to one another in staggered manner in order to thereby achieve higher working speeds of the textile machine and more positive procedures for the selection of the working elements. However, this arrangement has the disadvantage that a lot of space is required and, in particular, that a large number of permanent magnets and control coils have to be used, which also substantially complicate machines of this type as regards their manufacture, installation and servicing and give rise to a greater susceptibility to failure.
A textile machine is known from the CH-A-479 738, in which a respective individual permanent magnet is provided for each needle or for each selection sinker, each of which permanent magnets is compensated in a controllable manner as regards its permanent-magnet-effect by means of an appropriate excitation coil in order to bring about the selection of the needles. A selection device of this type is very expensive due to the large number of individual excitation coils and the expense associated therewith for the respective control means and the cabling for the excitation coils. In addition, the biassing for the sinkers by means of springs and the components for the release and arresting of the sinkers is very complicated and expensive. Moreover, the temperature of the needle bed is raised due to the excitation of the coils, which are effectively in thermal contact therewith, which is something that is disadvantageous.
A textile machine is known from the WO-A-9 202 672, in which an electromagnetic control system is guided over a needle bed, wherein respective static intermediate elements consisting of ferromagnetic material are provided in the needle bed for each individual needle channel or for each needle or sinker, in an area that is basically not magnetically excitable, in order to conduct a magnetic flux through these intermediate elements to the needles or to the sinkers. The manufacture of a control system of this type or, of a needle bed of this type is expensive due to the static intermediate elements in the needle bed.